Electronic devices have been reduced in size and weight in recent years, and there has been an increasing market demand for switching power supplies with increased efficiency and reduced size and weight. For example, in the market for flat screen television sets or the like, in which output current ripple characteristics are relatively moderate, a current resonant half bridge converter in which a sinusoidal resonant current is made to flow in a transformer to cause the transformer to operate while utilizing an LC resonance phenomenon is being put to practical use while taking advantage of its feature of being highly efficient.
For example, Japanese Unexamined Patent Application Publication No. 9-308243 discloses an LC series resonant DC-DC converter. FIG. 13 is a basic circuit diagram of a switching power supply device described in Japanese Unexamined Patent Application Publication No. 9-308243. This switching power supply device is a current resonant half bridge DC-DC converter, in which an LC resonant circuit that is formed by an inductor Lr and a capacitor Cr and two switching elements Q1 and Q2 are connected to a primary winding np of a transformer T. A rectifying smoothing circuit that is formed by diodes D3 and D4 and a capacitor Co is formed on secondary windings ns1 and ns2 of the transformer T.
With the configuration described above, the switching elements Q1 and Q2 are complementarily turned on and off with a dead time, and the waveform of a current that flows through the transformer T thus has a sinusoidal resonant waveform. In addition, electric power is transmitted from the primary side to the secondary side during both on periods and off periods of the two switching elements Q1 and Q2.